The Distinctive and Common Qualitiesof Job Shops and Flow Shops

ABSTRACT

The purpose of this paper is to clarify the similarities and differences between job shops and flow shops as well as advise with regard to facility layout. The selection of a job shop or flow shop layout should be made based upon the properties and characteristics the manufacturers desire to have in place for producing their products. These considerations will ultimately result in cost-saving measures as well as increases in overall efficiency.

I. Explanation of the Defining Attributes

Elements of Job Shops

A production environment in which similar operations and equipment are organized by function is typically referred to as a job shop. Job shops are a unique form of manufacturing in which the jobs pass through functional departments and lots. Within each lot may, a variety of different operational routing may exist. Several internal constraints on jobs and machines present themselves in this layout: No precedence constraints abide among tasks of different jobs; interruptions of tasks are not allowed; and each job can be performed only on machine at a time (1).

A particularly distinguishing feature of the job shop is its functional design. Design by process is a feature that commonly contributes to the higher customization of products. Such diverse products include space vehicles, aircraft, machine tools, and specialized tools and equipment. Due to the wide range of services provided, equipment that can perform a broader range of tasks is required and workers must have high skill levels to perform the assigned tasks. The layout tends to be less capital intensive and more reliant upon higher workforce wages (2).

Machines are grouped according the type of manufacturing process. The lathes, drill presses, and machines capable of plastic molding are grouped separately throughout the facility. Route sheets are used as the production control device to define the path of the material through the manufacturing system. Materials tend to be transported from one machine to the next on forklifts and handcarts since there the actual quantity of items produced will be relatively low as compared other layouts (3).

As production increases many job shops become production shops, where products begin to be created in large batches. Batches are produced to satisfy customer demand for an item. When items produced exceed demand, the shop builds an inventory of the item and changes over machines to fill other orders (4). In the job shop parts spend approximately ninety-five percent of their time waiting or being transported and only a very minute five percent of the time are they actually utilized (3).

Layout of the Job Shop

The typical job shop design consists of workers, machine tools, machine tool operators, and parts to be manufactured. Variables associated with job and flow shops production consist of number of jobs, quantity of machines in the shop, the job arrival rate, the processing time, total number of parts required, the due date, and the actual time to for the job to be accomplished. Job shops come in many different forms. The classical job shop is characteristically the simplest type of job shop, whereas the flexible job shop is noted for being the most complex of the various types (5).

FIGURE 1: THREE PRODUCT JOB-SHOP LAYOUT

In a job shop, the facility is laid out by function. The above figure displays three different product types making their respective paths through the facility. Ideally, of course, each of these products could randomly bounce around from job to job in a pure job shop. In reality, however, there is at least some degree of dependence from job to job.

Types of Job Shops

Job shops come in many different variations. One of the most important characteristics of the classical job shop is that manufactured parts acquire value in chronological order. Once the worked-upon part reaches its final work cell in the job shop layout, the manufactured part is ready to be utilitzed (6). Recirculation may also occur in job shops. This particular happenstance occurs when the part is routed to a machine it has already visited and the job is performed more than once. The flexible job shop often times synergizes the machine work. Work is performed on more than one part simultaneously. This leads to higher equipment utilization time and ultimately provides a higher degree of flexibility for those who have planned the facility layout (7).

Flow Shop Defined

The flow shop has a product-oriented layout composed of mainly flow lines. Material is carried through the shop on conveyors, moving belts, and transfer devices. The flow shop consists of a specified amount of machines in a series and each job has to be processed systematically as the machines are set up sequentially. The route schedule is quite rigid and bottlenecking is a common problem (8).

The flow shop design is similar to a queue network. In a queuing network jobs can not be released until the preceding operation is finished. Flow shops follow the First In First Out (FIFO) discipline which simply states no job can pass another job while waiting in a queue. After the completion of one job, another manufactured part joins the queue (9).

Flow shops have very high production rate and are often used in mass production. Plants that utilize the flow shop design may often be designed to manufacture one product or a comparatively small family of products. To accommodate the process of manufacturing special products plants have to invest in specialized equipment. Considering that skills are transferred from the operator to the machine in this type of layout, workforce skill levels in flow shops are relatively lower than those in job shops, but machinery is much more specialized and costly (5).

FIGURE 2: THREE-PRODUCT FLOW-SHOP LAYOUT

Above is an example of a product-oriented layout, which is in this case a flow shop. While some of the lines contain the same machinery, the flow lines never cross. Flow shops always have the same starting and ending points as well.

Types of Flow Shops

The no-wait flow shop requires that once a job has started it must be processed until completion without any interruption either on or between machines. This implies that the starting time of a job at the first machine has to be delayed to ensure that the